Three-dimensional structures with micron-scale features have many potential applications, for example as photonic band gap materials, tissue engineering scaffolds, biosensors, and drug delivery systems. Consequently, several assembly techniques for fabricating complex three-dimensional structures with features smaller than 100 microns have been developed, such as microfabrication, holographic lithography, two-photon polymerization and colloidal self assembly. However, all these techniques have limitations that reduce their utility.
Two-photon polymerization is capable of creating three-dimensional structures with sub-micron features, but from precursors that are not biocompatible. Many techniques have been developed to fabricate three-dimensional photonic crystals, but they rely on expensive, complicated equipment or time-consuming procedures. Colloidal self-assembly has also been utilized to make three-dimensional periodic structures, but controlling the formation of defects is difficult.
One fabrication technique relies on the deposition of viscoelastic colloidal inks, usually by a robotic apparatus. These inks flow through a deposition nozzle because the applied pressure shears the interparticle bonds, inducing a breakdown in the elastic modulus. The modulus recovers immediately after leaving the nozzle, and the ink solidifies to maintain its shape and span unsupported regions. The particles in the ink have a mean diameter of about 1 micron, meaning that it would be impossible for the ink to flow through a 1 micron diameter deposition nozzle without clogging or jamming. In practice, nanoparticle inks (mean diameter˜60 nm) also tend to jam nozzles smaller than 30 microns, limiting the applicability of viscoelastic colloidal inks to this length scale.
Polymeric solutions are used in nature to fabricate thin filaments. Spiders, for example, derive their silk fibers from a concentrated protein biopolymer solution that solidifies as it is drawn to form an extremely strong filament. The extensional flow of the solution aligns liquid crystal sheets in the polymer, and the solution gels by adding ions as it leaves the spinneret. This process was artificially recreated by the deposition of the recombinant spider silk biopolymer into a polar “deposition bath” to produce filament fibers with comparable properties.